INTRODUCTION. 21 



grain can grow, so by the special germ for each disease 

 only that disease can be produced. 



He found in all decomposing matters innumerable 

 minute " animalculse," and was so firmly convinced of 

 their etiological relation to the process that he formu- 

 lated the law : that decomposition can only take place 

 when the decomposable material becomes coated with a 

 layer of the organisms, and can proceed only when they 

 increase and multiply. 



However convincing the arguments of Plenciz may 

 appear, they seem to have been lost sight of in the 

 course of subsequent events, and by a few were even 

 regarded as the productions of an unbalanced mind. 

 For example, as late as 1820 we find Ozanam express- 

 ing himself on the subject as follows : " Many authors 

 have written concerning the animal nature of the con- 

 tagion of disease ; many have indeed assumed it to 

 be developed from animal substances, and that it is 

 itself animal and possesses the property of life ; I 

 shall not waste time in effort to refute these absurd 

 hypotheses." 



Similar expressions of opinion were heard from many 

 other investigators of the time, all tending in the same 

 direction, all doubting the possibility of these micro- 

 scopic creatures belonging to the world of living things. 



It was not until between the fourth and fifth decades 

 of the nineteenth century that by the fortunate coinci- 

 dence of a number of important discoveries the true rela- 

 tion of the lower organisms to infectious diseases was 

 scientifically pointed out. With the fundamental inves- 

 tigations of Pasteur upon the souring and putrefaction of 

 beer and wine ; with the discovery by Pollender and 

 Davaine of the presence of rod-shaped organisms in the 



